1. Field of the Invention
The present invention relates to a steering wheel, including an air bag device, that has a very soft outer cover and yet includes a membrane type horn switch in an exterior cover, located at a position between the air bag and the outer cover.
2. Description of the Prior Art
A conventional steering wheel with an air bag device is disclosed in Japanese Patent Publication No. Hei. 6-305427. This steering wheel has essentially an annular steering ring portion, a boss portion located at the center of the ring portion, for connection to the upper end of a steering shaft. Spoke portions extend between the ring portion and the boss portion. The air bag device is mounted on the boss portion.
The steering wheel has an exterior pad, including an assembled air bag and an inflator, and a steering wheel body that comprises the annular ring portion, the spoke portions and the boss portion.
The steering wheel body is comprised of a core portion and a cover portion made of a soft synthetic resin. The core portion is comprised of a ring core located in the annular ring portion, spoke cores located in the spoke portions to be coupled to the ring core, and a boss core located in the boss portion to couple the spoke cores together. The boss core is connected to the steering shaft which is inserted from below and secured at its upper end by means of a nut.
The cover portion covers not only the ring core but also the spoke cores each extending from the ring core to the nearby area of the pad. The pad, while covering the air bag device, is secured by being screwed to the spoke cores and the boss core which are components of the steering wheel body.
This conventional steering wheel with an air bag device is so adapted that the pad is mounted on the steering wheel body from above the boss portion and fixed by securing means such as screws. A conspicuous parting line appears on the upper side of the steering wheel on the spokes as a border between the center steering wheel body and the pad on the annular ring. This conspicuous parting line impairs the design of the steering wheel and is not desirable.
In order to assure that such a parting line is uniform around the periphery of the pad portion, the individual components must be assembled with high precision. This requires strict control over a variety of factors including the dimensional precision of the respective components and their assembly.
In addition, when the steering wheel is mounted on a vehicle, the steering shaft is secured to the boss core by means of a nut, with the pad then being removed. Thus, the pad is mounted to the steering Wheel. Thus, two steps, one being securing the steering wheel by means of a nut and the other, mounting the pad, are involved, thereby making it a time-consuming operation to fit the steering wheel on to the vehicle.
Meanwhile, Japanese Utility Model Publication No. Sho. 63-16267 discloses that the cover portion extends the upper part of the boss portion from the spoke portions and the boss core provides for a connecting portion which the steering shaft can connect. In short, no parting line will appear on the upper side of the steering wheel. Additionally, only the lower side of the steering wheel need be used to accomplish its mounting on the steering shaft and the steering wheel substantially can be connected to the steering shaft in a single step.
However, the steering wheel described in the above Publication does not include an air bag device and it is not easy to mount the air bag device on the boss portion.
Further, the provision of an air bag device requires that the cover portion should tear smoothly upon inflation of the air bag.
Another example of a conventional steering wheel with an air bag device having a membrane horn switch is described in Japanese Patent Publication No. Hei 6-305425, now U.S. Pat. application Ser. No. 08/216,824.
This steering wheel comprises an annular ring portion, a boss portion, located at the center of the ring portion, that is connected to the upper end of a steering shaft, and spoke portions for connecting the ring to the boss portions together. The air bag device is also mounted on the boss portion.
The boss portion includes a rigid support plate located over the folded air bag. A membrane type horn switch is located on the upper side of the support plate. An outer cover, made of a thermoplastic polyester-base elastomer, extends over the horn switch.
The outer cover is provided with weakened areas that define door portions that open when the air bag in the air bag device inflates. The horn switch and the support plate are located within the door portions.
The underside of the door portions of the outer cover are provided with a plurality of downwardly extending mounting projections. The support plate is provided with a plurality of mounting holes in correspondence with those mounting projections which are received in and engaged by respective mounting holes.
In a conventional steering wheel, the support plate is retained on the outer cover, made of a thermoplastic elastomer. The outer cover can retain the support plate by the outer cover itself since it has the necessary rigidity despite its comparative softness.
If the outer cover is formed of soft polyurethane, because the support plate has its own rigidity, the thermoplastic elastomer has rigidity (unlike soft polyurethane which is more elastic to provide a better feel than the thermoplastic elastomer), it is difficult to secure the necessary tensile strength by the soft polyurethane and the outer cover is no longer capable of retaining the support plate such that the membrane switch can be provided above the air bag.